The purpose of these studies is to study factors contributing to variability in stimulant and sedative drug response. The aims of the family history study are (1) to compare athe response to an acute alcohol challenge in individuals at low and high risk for substance abuse as defined by family history of alcoholism, (2) to compare the response to a stimulant drug (d-amphetamine) in the same groups, and (3) to examine the concordance of response to a stimulant and a sedative drug within subjects. Response to drug will be measured using a battery of measures including resting EEG, event-related potentials, the Continuous Performance Task, physiological tremor and standing stability, and self-reported subjective effects. The aims of the drug preference study are (1) to test the hypothesis that individuals who prefer d-amphetamine over placebo will report more stimulant like subjective effects, and (2) to test the hypothesis that individuals who prefer d-amphetamine over placebo will show behavioral and electrophysiological responses consistent with the stimulant-like subjective effects, while individuals who prefer placebo will have a different profile of behavioral and physiological effects. Response to drug will be measured using the same battery of measures as will be used in the family history study. The twins study of sustained attention will (1) permit estimates of the heritability of sustained attention, (2) examine developmental changes in that heritability, and (3) disentangle the effects of heredity and environment ont he observed gender differences in the Continuous Performance Test. Understanding of the genetic effects ont he CPT will help to clarify the role of attention deficits in substance abuse and other disorders.